Country flags seem like they could be used for political trolling.
Die faces could lead to weird rolling threads or other things.
Musical notes, you got me, can't really think of anything too bad for those.
The markup's not great, but too much formatting is distracting. I personally prefer the limited options. You focus more on the content of your comment than making it look pretty.
The only thing i really despise about hn's formatting is the code blocks or whatever they are, the one on mobile that vanishes off the side and you have to scroll horizontally to read everything. I really can't stand when people use those for quotes.
Other than that though, hn's formatting makes everything uniform and fairly easy to read through. There's no fancy nonsense getting in the way of things.
Actually, that's part of why those code block things piss me off, they're probably the fanciest piece of formatting you can do and all it does is obstruct information and make me waste time while reading.